Dielectric fluids are used in capacitors, transformers, co-axial cables and other types of electrical apparatus to exclude gases from the insulating portion of the apparatus and thereby provide an insulation of higher voltage capability.
A good dielectric fluid should have a high electric strength, and be inexpensive and, preferably fire-resistant. It should be easily purifiable to achieve a low conductivity or dissipation factor. If it is used in capacitors, it should have a relatively high dielectric constant, and should be capable of impregnating the dielectric barrier of the capacitor. It should have good corona properties, especially if used in capacitors. Good corona properties include high corona (partial discharge) starting and extinction voltages in a test where the voltage is increased to the onset of corona and then decreased. Also, severe corona (which can occur sporadically in service) should not produce permanently damaging effects, such as persistent bubbles, which would depress considerably the corona extinction and restart voltages. The dielectric fluid should maintain its good electrical properties at high temperatures in the environment of the dielectric and metallic components of the device. It should not be too volatile since volatile liquids are difficult to handle and evaporate rapidly in service. When a dielectric fluid solidifies it cracks and loses electric strength. Therefore, a good dielectric fluid should have a low solidification or cracking temperature so that it can be used at low temperatures.
At the present time chlorinated diphenyl compounds such as trichlorodiphenyl (hereinafter "TCDP") are widely used commercially as dielectric fluids as they offer a very good compromise of these many desirable properties. However, dielectric fluids are used in large quantities and there is always the danger of an accidental spilling or leaking of the fluid. Many of the chlorinated compounds are not readily biodegradable but persist for long periods of time, accumulating in the food chain. Therefore, in spite of their desirable properties, it is quite possible that chlorinated diphenyls may soon be banned entirely for use as dielectric fluids or such precautionary practices may be imposed that their use would be very costly. Because environmental considerations have in recent years become a very important consideration in selecting dielectric fluids, great difficulty has been encountered in finding satisfactory substitutes for these halogenated dielectric fluids.